A Message in a Bottle, Thrown Hard

I have been carrying this idea around for a long time, and it started with a question I could never get past: where does the money come from?

Not government money. I mean the money for the kind of program this magazine keeps describing, the one that mines the rock and prints the parts and plants the satellites and finally spins up a habitat. Somebody has to pay for the first ten thousand things before anything out there pays for itself. And every plan I've ever read either hand-waves that part or assumes a billionaire.

Here's what I've wanted to do instead. Sell people a piece of the sky.

The product

You order a capsule. It arrives in the mail. It's a small metal cylinder that unscrews in the middle, and the size is whatever turns out to be easiest to handle, probably somewhere between a pill bottle and a thermos.

You open it and you put your things in it. Photographs of your family. A letter to whoever finds it. Your grandfather's pocketknife. A phone with your pictures on it. A drawing your kid made. Whatever you'd want to send to the future, or to somebody who isn't from here.

You screw it shut and mail it back. We put it on a rocket. And once that rocket is up there and moving, we throw it. Not into orbit. Out. Away from the Earth, on a path that never comes back, so that your things are traveling through space for as long as space is there.

That's the whole product. You are paying to have something of yours leave the planet forever, and the money you pay funds the program that builds the launcher.

I want to be clear about one thing up front, because I thought about it and decided against it. I'm not going to talk about sending remains. There's an existing business that does that and has since the nineties, and it brings a set of rules and feelings with it that I don't want tangled up in this. What I'm describing is a message, not a funeral. A note in a bottle, thrown hard.

This has been done before, twice

Neither of these was a business, but both prove the instinct is real.

In 1977, NASA bolted a gold-plated phonograph record to each of the two Voyager probes. Carl Sagan's committee spent almost a year choosing what went on it: about 115 images, natural sounds, music from around the world, and greetings in 55 languages, meant for whoever might find the spacecraft in interstellar space. ¹ NASA itself calls the record a time capsule. ² Neither Voyager is aimed at any particular star, and the closest approach either one makes to another system is 1.6 light-years from a star called Gliese 445, roughly forty thousand years from now. ³

The record did nothing for the mission's science. It cost extra time and money and it wasn't practical at all. They did it anyway, because people want to send something. ⁴

Then in February 2018, SpaceX flew the first Falcon Heavy and needed a dummy payload. Elon Musk sent his own Tesla Roadster, with a mannequin in a spacesuit at the wheel, a toy model of the car on the dash, a disk of Isaac Asimov's Foundation novels, and a plaque with six thousand employee names. ⁵ The upper stage parked in Earth orbit for about six hours, then lit its engine one last time and pushed the whole thing out of Earth's gravity. ⁶ The car is now in an orbit around the Sun that swings out past the orbit of Mars, and it's expected to stay there for millions of years. ⁷

That's the closest thing to a space time capsule anybody has launched. It got there on a single extra engine burn. And it wasn't cheap: it took the most powerful rocket flying at the time to do it.

That's exactly the gap I want to fill. The Roadster proved people will fill a capsule with their own stuff and send it off. What it didn't do is make that affordable for anybody who isn't Elon Musk.

The launcher is the cheap part

Getting to orbit is the expensive part of any launch. Once you're up there and moving at orbital speed, the extra push to leave the Earth altogether is comparatively small. From low Earth orbit, you need something like another 3.2 kilometers per second to escape Earth's gravity for good. ⁸

Now, 3.2 kilometers per second is a lot to ask of a chemical rocket, but it's a wonderful number for a magnetic launcher, for one reason: the payload doesn't care about g-forces.

Everything I've written about the mass driver has been shaped by what a payload can survive. Freight can take hundreds of g's. People can't. A sealed can of photographs can take just about anything. A 3.2 km/s throw down a rail a hundred meters long works out to a bit over five thousand g's, and down a thirty-meter rail it's more like seventeen thousand. Either one would turn a person into paste. A capsule doesn't notice. It's a lump of metal with paper inside.

So the launcher for this product is a small coil gun, the same kind I built in my garage in 1994 with ten magnets and a starter relay, scaled up to a few tens of meters. It's cheap by space standards because it never has to be gentle. It doesn't need the Hawaii tunnel. It doesn't even need a station. And that last part is where this idea gets practical, so let me take it in order.

Throw it from the ship

The simplest version of this needs no coil gun at all, and SpaceX already flew it. The Roadster left Earth because the upper stage that carried it up was still attached, still had propellant, and fired one more time. ⁶ Do the same with a rack of capsules instead of a car. Buy a ride on somebody's launch, ride the upper stage into orbit, and let its final burn throw the whole rack at once. Every capsule on that rack leaves on the same path, which is fine for a first product. You're selling "your things left the Earth," not a custom trajectory.

That's stage one, and it makes money before we own anything but a rack and a mailing list.

Stage two is a dedicated ship. Not a station, a ship: it goes up with a load of capsules and a coil on its back, throws them one at a time in whatever directions the customers paid for, and comes home. Here the coil gun earns its keep, because every throw can be different. One capsule toward the outer planets, the next into a high Earth orbit, the next timed for a window. The rack-and-burn version can't do that.

A ship is also the right answer to the thing I worried about most, which is recoil.

What the throw does to the thrower

Every time you throw something one way, the thing you're standing on gets pushed the other. There is no way around it. So I did the arithmetic. Take a two-pound capsule, about nine-tenths of a kilogram, and throw it at 1 km/s off something the size of the International Space Station, roughly 450 tons. The station moves by about two millimeters per second. Throw it at the full 3.2 km/s and it's about six and a half millimeters per second. You'd need something like 150 capsules at full speed before you'd cost the station one meter per second.

For scale, the station already loses up to a hundred meters of altitude a day to the thin atmosphere at 400 km, and gets pushed back up roughly once a month by a burn of a meter or so per second. ⁹ So a few hundred capsules a year adds a meaningful line to the reboost budget, but it's the same kind of line that's already there. Throw along the direction of travel and you mostly change altitude, which is what reboost corrects anyway; alternate directions and the kicks largely cancel. You price it in. The recoil isn't a flaw. It's the ticket.

On a ship it isn't even a line item. A ship has engines and propellant by definition, it's going to burn to come home anyway, and it doesn't have to hold its position for twenty years. It throws, it drifts a little, it corrects. Nobody cares if the throws don't balance.

The station-mounted rail is stage three, and it's where this goes when the volume is high enough that you'd rather not fly a ship for every batch. By then the station exists for other reasons, the coil is bolted to it, and the recoil is a known cost with a known fix. Three stages, and there's revenue at every one of them.

Three ways to go, three prices

Once you accept that the launch is the easy bit, the product turns into a menu, and the menu is really a set of speeds. This is where the customer chooses how far their things go, and the physics sets the price.

Near. Throw the capsule just hard enough to reach a high, stable Earth orbit. It circles the planet for centuries. Cheapest tier, most modest promise. Your things are up there, and if you point a telescope right you could in principle see them go by.

Far. Throw it at escape speed. It leaves the Earth for good and settles into an orbit around the Sun, like the Roadster. It never comes home, but it never leaves the neighborhood either. It crosses between the planets forever. This is the honest middle tier, and it's the one I'd sell most of.

Gone. Leave the Sun. This is the promise everybody wants to make, and it's a different order of problem. Escaping the Sun from low Earth orbit takes roughly 8.7 kilometers per second, nearly three times the Earth-escape figure, and only a handful of probes have ever done it, most of them with a gravity assist from Jupiter. ⁸ You can't buy a Jupiter flyby with a bigger coil. You have to wait for the planet to be in the right place.

So the honest version of the top tier is this: it costs more, and it also has a launch window.

The sail

There's a way to make the top tier real without waiting on Jupiter, and it fits this product better than it fits almost anything else. Put a sail on the capsule.

Sunlight pushes. It pushes very gently, which is why sails are useless for anything in a hurry. JAXA's IKAROS unfurled a 196 square meter sail on the way to Venus in 2010 and became the first craft measurably accelerated by sunlight alone. ¹⁰ The push on that entire sail was well under a millinewton. But your capsule has the one thing that makes a sail worthwhile: nobody is waiting for it. It can accelerate for a hundred years.

Here's where I have to give you the number that bit me.

Whether a sail can escape the Sun on light alone comes down to one ratio: sunlight's push against the Sun's pull. Both fall off with distance in exactly the same way, so the ratio never changes, and if it's less than one the Sun wins forever. To get it above one, the total weight of sail and cargo together has to come in under about 1.5 grams for every square meter of sail. For a two-pound capsule, that means a sail of roughly 600 square meters, about 24 meters on a side, even if the sail itself weighed nothing.

And it doesn't weigh nothing. IKAROS's film was 7.5 microns of polyimide, which works out to something like 10 grams a square meter. ¹⁰ That's already six times over the limit before you've hung a capsule on it. Thinner films exist in the lab, around a gram a square meter, but even those can't break the ratio on their own. A sail heavy enough to be built is too heavy to escape.

So a sail alone doesn't buy you the "gone" tier. What it buys is something I like almost as much. A modest sail, say ten meters square, on a two-pound capsule won't leave the Sun, but it will slowly spiral outward for as long as it exists. The "far" tier becomes a capsule that keeps drifting farther every year. Not gone, but going.

There's one more option, and it's the one I'd actually build toward. Instead of the Sun, push the sail with a light source on the ship or the station that threw it. A laser aimed at the capsule as it leaves gives it a shove that doesn't weaken just because the capsule is heavy, and it keeps pushing for as long as you can hold the beam on it. That turns the thrower into the engine and lets the capsule stay a dumb metal can with a sheet on the back. The "gone" tier stops being a launch window and becomes a matter of how much power you're willing to spend.

Who finds it

I said this was a message, not a funeral, so let me say what the message is for.

The odds that anyone ever opens one of these capsules are astronomically small. The Voyagers have been flying for nearly fifty years and won't pass close to another star for forty thousand more. ³ Space is that empty. A capsule thrown from Earth orbit will almost certainly drift, undisturbed, for longer than our species has existed.

That's not an argument against doing it. The Golden Record team knew all of that and did it anyway, and Sagan said the point wasn't really the aliens, it was what launching the bottle said about us. ⁴ I feel the same way about a family's photographs. The value isn't in being found. It's in the throw.

But I'd design for finding anyway, because that's the honest way to build it. Metal, not plastic. No batteries, nothing that decays. Photographs printed, not just on a phone, since a phone is a brick in twenty years and a print lasts. A simple pictorial cover, the way Voyager's record explained itself in scratches. And a small piece of the capsule's exterior stamped with a date and a direction, so that if it's ever picked up, the finder knows where it came from and when.

What it pays for

Here's the part I'm least sure about and most interested in.

I don't know what people would pay. I know they paid for a plaque on the Roadster's adapter plate, in a sense, with six thousand names on it. I know they'll spend thousands to send remains up on somebody else's rocket. And I know the cost per capsule, once the rack is riding up anyway, is close to nothing: a little power, a little propellant, a little handling.

That gap between what it costs to throw and what people would pay to have it thrown is the space program's revenue. And unlike selling asteroid metal or renting habitat space, it works from day one, before anything out there produces anything, because it's selling something Earth already has: people who want their story to outlast them.

The question

So, three questions this time, because this one isn't finished.

Would you buy one? And if you would, which tier, and what would you put in it?

If you're an engineer: what have I got wrong about the recoil, or the sail, or the rail? I ran the numbers by hand and I'd rather be corrected here than in orbit.

And a wider one. This magazine has spent a dozen articles on how to build a space program. Is there a better way to pay for one than this? If you've got an idea, I want to hear it.

Write to hello@starboundmagazine.com.

Ideas in Star Bound are published open — take them, build them better. Our open proposal.

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A note on where this comes from. I am not a metallurgist, a spacecraft engineer, or a degreed engineer of any kind. But I am a practical one, with more than forty years in heavy industrial construction: welding, millwright work, hydraulics, mining, and thirty-three years of electrical construction and commissioning on the North Slope of Alaska, in a place that punishes anything built wrong. Much of that time I have worked as a field engineer, which means I am the one standing in front of the equipment when the drawing turns out to be wrong, and the fix has to be made on the spot, with what is on hand, and has to work the first time. Anyone who has done that long enough learns that a lot of engineering gets corrected in the field by people with more experience than the people who drew it. That is the eye I bring to these subjects: not what should work on paper, but what actually gets built, and what breaks. The facts I take from other people are footnoted so you can check them and read further. The ideas built on top of those facts are my own, and I would rather see them knocked down by somebody who knows better than left standing because nobody looked.

Sources

1. NASA Science, "Golden Record Images" and "Golden Record Greetings" — the Sagan committee's selection of images, sounds, music, and 55-language greetings. ↩
2. NASA/JPL Voyager site, "The Golden Record" — NASA describes the record as a time capsule. ↩
3. "Voyager Golden Record," Wikipedia — no target star; closest approach 1.6 light-years from Gliese 445 in about 40,000 years. ↩
4. The Planetary Society, "The Voyager Golden Records: A cosmic love letter" — the record's lack of scientific purpose and Sagan's "bottle into the cosmic ocean" framing. ↩
5. "Falcon Heavy test flight," Wikipedia — the Roadster's cargo: Starman, the Hot Wheels model, the Arch Mission Foundation disk, the employee plaque. ↩
6. Space.com, "Skywatchers Spot Falcon Heavy Leaving Earth's Orbit," February 2018 — the six-hour coast and the third burn out of Earth orbit. ↩
7. "Elon Musk's Tesla Roadster," Wikipedia, and Orbit Codex — heliocentric orbit reaching 1.66 AU, 557-day period, expected to persist for millions of years. ↩
8. Editor's figures from standard orbital mechanics: roughly 3.2 km/s from low Earth orbit to Earth escape, and roughly 8.7 km/s from low Earth orbit to solar-system escape via direct burn. Any introductory astrodynamics text will confirm both; the exact values depend on the starting orbit. ↩
9. Universe Today, "Watch What Happens to Astronauts When the ISS Gets an Orbital Reboost" — altitude loss of up to 100 m/day, reboosts about monthly, a cited 12-minute burn of 1.34 m/s. ↩
10. "IKAROS," Wikipedia — 196 m² sail, 7.5-micron polyimide film, solar-pressure acceleration confirmed July 2010. ↩

The recoil, rail-g, and sail-ratio figures in this article are my own calculations from the cited inputs (450-ton station, 0.9 kg capsule, 1,361 W/m² sunlight at Earth's distance) and are offered to be checked. The capsule concept and the coil-gun launcher are my own.